This invention relates to a time axis adjusting system for use in a radio communication network for carrying out time division multiple access (TDMA) communication.
As described in U.S. Pat. No. 4,330,859 issued to Masami Takada and assigned to the present assignee, a multi-directional time division multiplex communication network comprises a central or base station and a plurality of substations around the central station. In a large-scale network, a plurality or repeater stations are situated around the central station with a plurality of substations scattered around each repeater station. The substations and/or the repeater stations will be called satellite stations in the following.
As will later be described more in detail with reference to one of nine figures of the accompanying drawing, a downward stream of transmission signals is sent from the central station towards the satellite stations in frames. Each frame comprises a predetermined number of time slots, which may be identified by consecutive numbers without loss of generality.
On carrying out communication with a selected one of the satellite stations, the central station assigns or allots one of the time slots in each frame to the selected station as a downward communication channel. The selected station may either be a call originating station or a satellite station to which a call originates at the central station. It is of course necessary on assigning the time slot that at least one of the time slots be idle in a frame, namely, not busy. The central station sends burst signals through the downward communication channel to the satellite stations as the transmission signals.
Clock synchronism is established in the satellite station by regenerating a sequence of clock pulses from the transmission signals. Furthermore, a time axis specific to the satellite station is generated by detecting a frame synchronization code included in the downward stream preferably in each frame.
The central station receives burst signals as an upward stream of transmission signals from those of the satellite stations which are communicating with the central station. Like for the transmission signals of the downward stream, time slots are assigned to the communicating satellite stations in each frame as upward communication channels. The time axis specific to each communicating satellite station must be in correct synchronism with the upward stream received at the central station.
According to the prior art, the time axis is adjusted in each satellite station so as to achieve the correct synchronism either by the use of a certain one of the communication channels or an additional channel. The prior art time axis adjusting methods are defective in the following respects.
When the communication channel is used, the time axis adjustment is carried out at the time of installation of the central and the satellite stations. The time axis must furthermore be adjusted at regular intervals with the communication interrupted. In other words, the time axis adjustment has a higher priority than the communication. Despite the higher priority, the adjustment must be carried out in short intervals of time. This is specifically objectionable when a new satellite station is added to the network. Moreover, the communication temporarily becomes infeasible due to the time axis adjustment. The network is therefore unreliable. In addition, the time slots are not effectively used in communication. This degrades the service.
When the additional channel is used, an additional radio frequency must be allotted to the network in question. The frequency bands are not effectively used. Furthermore, each satellite station must include an additional receiver. The network is therefore rendered expensive.